1. Field of the Invention
The present invention relates in general to an apparatus for generating a DC high voltage for application to an X-ray tube. More particularly, this invention is directed to a DC high voltage generator employing a series, resonance type, bridge inverter suitable for use in a diagnostic X-ray apparatus.
2. Description of the Prior Art
A need to produce stable X-rays is strongly felt in the field of diagnostic X-ray apparatus. A major solution aimed at achieving such stable X-ray generation is that of continuously applying a stable DC high voltage, which does not contain ripple components, to an X-ray tube. Such a DC high voltage generator employing a series, resonance type, bridge inverter is known from, for instance, U.S. Pat. No. 4,225,788 to Franke, issued on Sept. 20, 1980.
The conventional DC high voltage generator rectifies and smoothes an AC voltage supplied from a power source (in general, a commercial power source) so as to obtain a DC voltage. The obtained DC voltage is alternately switched by series-connected first and second thyristors, and the switching voltages are applied to a resonant circuit consisting of a resonance capacitor connected in series with the primary winding of a transformer, thereby inducing attenuated oscillation. Thus, a DC high voltage for an X-ray tube is obtained based upon the AC high voltage induced in the secondary winding of the transformer.
The level of the high X-ray tube voltage in the DC high voltage generator can be varied by changing turn-on periods of the first and second thyristors. However, when the high X-ray tube voltage is varied by changing the turn-on periods of the thyristors, the following problems occur. FIGS. 1 and 2 are waveform charts showing the relationship between a gate pulse of the thyristors and current flowing through a transformer winding upon switching the first and second thyristors. Referring to FIGS. 1 and 2, "a" indicates current flowing in the turn-ON state of the first thyristor, and "b" indicates current flowing upon turning ON the second thyristor. Note that a hatched portion represents current flowing in an opposite direction due to the attenuated oscillation of the series resonant circuit.
As shown in FIG. 1, when repetition periods of gate pulses GP1 and GP2, applied to the gates of the first and second thyristors, are short (at a maximum turn-on frequency), current continuously flows through the primary winding of the transformer. Therefore, the X-ray tube voltage is increased and ripple components contained in this higher X-ray tube voltage are relatively reduced.
However, when the repetition periods of gate pulses GP1 and GP2, applied to the gates of the fifst and second thyristors, are long (at a mininum turn-on frequency) as shown in FIG. 2, the current intermittently flows through the primary winding of the transformer. For this reason, the X-ray tube voltage is reduced. In addition, ripple components contained in this lower X-ray tube voltage are increased in comparison to those in the higher X-ray tube voltage.
Therefore, when the X-ray tube voltage is varied by changing the turn-on periods of the first and second thyristors, it is known that the ripple components increase as the X-ray tube voltage decreases. In this manner, if a large number of ripple components are contained in the X-ray tube voltage, the X-ray dose generated from the X-ray tube is reduced, and a good X-ray photograph cannot be obtained, a particularly disadvantageous result, in mammography which is performed in a low X-ray voltage range.
Another serious problem exists in that the variable range of an X-ray tube voltage may differ in accordance with a load, i.e., an X-ray tube circuit. FIG. 3 shows the variable range of tube voltage (kV) with respect to a turn-on period (.tau.) of the above-described thyristor, using respective tube currents as parameters. As can be seen from FIG. 3, as the tube current is decreased, i.e., in a low load state such as fluoroscopy, the tube voltage variable range is narrowed. On the other hand, when the tube current is large, i.e., in a heavy load state such as X-ray radiography, the tube voltage variable range is widened.
Conventionally, a DC/DC converter and other phase controls using a thyristor have been proposed as methods for varying an input voltage to a transformer so as to vary the tube voltage. However, in this case, the number of circuit elements is increased, resulting in a large, expensive apparatus, and generation of noise.